Add initializer to set sourceLayer in MGLStyleLayer

[boundsj]

In the GL JS example for adding a third party vector source and also in this Android demo app example the source-layer property can be set to a specific layer.

The iOS MGL*StyleLayer classes do not offer a way to set this value either in the initializers in the MGLStyleLayer protocol or via a property or setter method. This feature is important for styling sources that have many layers.

cc @1ec5 @frederoni

[jfirebaugh]

Note that the core setter isn't hooked up internally to cause a refresh, so it only works when called prior to adding the source to the style. I don't foresee any technical blocker to allowing on-the-fly mutation, but it's probably not a big priority (GL JS doesn't have such an API for example).

[boundsj]

Thanks for pointing that out @jfirebaugh. I adjusted the title of this issue and also proposed a fix (to add an init method only, no setter) in #6269